— The suspects were brothers, a terrorism expert briefed on the investigation said.

— The suspects are originally from the Russian Caucasus and had moved to Kazakhstan at a young age before coming to the United States several years ago, according to a source briefed on the investigation.

— The violence began late Thursday night when a Massachusetts Institute of Technology police officer was shot and killed when he responded to a disturbance on campus. The slain officer was identified as Sean Collier, 26, of Somerville, Massachusetts, law enforcement officials said.

— Another policeman, transit officer Richard H. Donohue Jr., was shot and wounded overnight. The 33-year-old is a three-year veteran of the force, a transit police spokesman said Friday.

— Within an hour, police were called to respond to a carjacking, which led them on a chase of the two suspects through Watertown.

— The fleeing men allegedly used explosives against police as they were being chased.

— Police have warned Watertown residents to lock their homes and stay away from windows and doors.